The present invention relates to coating compositions which are based on poly(vinyl alcohol) ("PVOH") and which provide enhanced oxygen and flavor/odor barrier characteristics.
Attempts have been made in the past to produce waterborne adhesive coating compositions including PVOH for use on hydrophobic polymeric substrates. For example, one PVOH containing coating is obtained by reacting PVOH with isocyanate or blocked isocyanate to form a coating which is adherent to polyolefins, but also contains residual, ureacted toxic isocyanates. As a result, such coating cannot be used in food packaging applications.
Certain polymeric films employed for the packaging of foods inherently permit the transmission of oxygen from the atmosphere to the inside of the packaged food. Oxygen causes the spoilage of packaged food by oxidation.
PVOH coatings, have been applied to various substrates in the past, and are known to provide a barrier to the transmission of oxygen. In certain applications, such as condiments packaging, PVOH has been used to prevent the flavor of condiments from passing through the packaging. In other applications, PVOH containing polymeric films have been used to protect the packaged food contents against acquisition of odors or even other flavors from the outside the packaging.
Polyvinyl alcohols are polymers containing (--CH.sub.2 --CHOH--) groups. They can be applied from water solution and are in many respects attractive materials for use as oxygen and flavor/odor barrier coatings for thermoplastic polymer substrates. However, PVOH is soluble in water and, as a result, susceptible to attack by moisture. The poor water resistance of PVOH restricts its use as a barrier coating to those few applications in which nearly anhydrous conditions prevail. Moisture may act to cause staining or wear due to friction, an unpleasant feel to the touch and a dull appearance. It is also known that oxygen and flavor/odor permeability of PVOH coatings increases in proportion to its moisture content.
A number of methods have been known for increasing the water resistance of PVOH, but none provides entirely satisfactory results. In spite of various heretofore practiced methods, PVOH still tends to soften, swell and lose adhering strength when exposed to water or even moisture vapor. When applied to hydrophobic surfaces, such as polyolefins, coatings including PVOH do not adhere well and are not moisture resistant. As a result, the polymeric substrate surface must be coated with a primer coating first and then covered with the PVOH containing coating. Thus, there is not presently available a single coating composition which acts as both a primer and a barrier. The coatings known to date which are used to increase or enforce barrier characteristics generally require a primer to improve adhesion or binding between the barrier coating and the film.
Accordingly, a coating composition with high oxygen and flavor/odor barrier which is moisture resistant and is directly applicable to hydrophobic substrates without the necessity of a primer coating is highly desirable.
It is, therefore, an object of the present invention to provide a PVOH containing coating composition which has high oxygen and flavor/odor barrier characteristics, is moisture resistant and is directly applicable to hydrophobic polymeric substrates without the requirement of a primer coating.
It is yet another object of the present invention to provide a PVOH containing coating composition which not only adheres tenaciously to hydrophobic polymeric substrates, but is also tacky and can be used as a primer coating for subsequently applied coatings.